A great many methods have previously been proposed for recovering oil from oil shale, nearly all of which involve some form of pyrolytic eduction. However, for a variety of economic reasons, none of these methods has yet proven competitive with the production of mineral oils from petroleum or other fossil sources. It may be said in general that the principal; overall difficulty involved in shale oil eduction resides in recovering essentially all of the hydrocarbonaceous material from the shale without resorting to prohibitively expensive methods. Since shale rock usually contains only about 20-80 gallons of oil per ton, it is a practical necessity to recover at least about 80-90 percent of this oil, and in the most economical possible fashion. It is not essential however that all of this oil be recovered as liquid product; combustible gaseous products can also be economically utilized in the eduction process itself, or in other ways. The overall objective remains to recover the maximum possible energy values at minimum expense.
Due to the expense involved in transporting raw shale rock and disposing of spent shale, it is a practical necessity in most eduction processes that the retorting facilities be located closely adjacent to the mining site. These mining sites are nearly always located some distance from refining facilities. Transporting the educed shale oil to such refining facilities has proven to be a problem, mainly because the high nitrogen content of shale oils renders them incompatible with crude petroleum and unsuited for transportation in common carrier pipelines. It would be highly desirable to provide at the retorting site at least sufficient refining facilities to convert the crude shale oil to a pipelineable material. The most desirable refining process for this purpose would be a catalytic hydrofining unit, but this has generally been considered impractical because of the expense involved in providing a suitable source of hydrogen. According to my process, a suitable such hydrogen source is integrated into the retorting system itself, thus providing the needed hydrogen at minimum expense.
Briefly, in my process, spent, coke-containing shale produced for example by the method described in my prior U.S. Pat. No. 3,361,644, is gravitated downwardly through a combusion-gasification zone, and a mixture of oxygen-containing gas and steam is passed upwardly, countercurrently therethrough. The amount of oxygen injected is controlled to burn only sufficient coke to produce the amount of heat required to initiate and drive essentially to equilibrium the endothermic gasification reactions such as:
______________________________________ 2CH.sup.1 ##STR1## 2C + H.sub.2 (1) C + 2H.sub.2 O ##STR2## CO.sub.2 + 2H.sub.2 (2) C + H.sub.2 O ##STR3## CO + H.sub.2 (3) C + CO.sub.2 ##STR4## 2CO (4) CO + H.sub.2 O ##STR5## CO.sub.2 + H.sub.2 (5) ______________________________________ .sup.1 "CH" is the approximate molar composition of the original coke on the spent shale. Reaction (1) is catalyzed by water vapor.
The resulting essentially oxygen-free off-gases will be designated herein as "water gas" even though that term conventionally refers to mixtures much richer in carbon monoxide and leaner in carbon dioxide than those produced herein.
The water gas from the gasification zone is then mixed with a major proportion of preheated process recycle gas comprising mainly carbon dioxide, hydrogen and light hydrocarbons derived from the retorting zone, and the resulting hot gas mixture is then passed downwardly, countercurrently to a stream of upflowing crushed shale rock in the eduction zone to effect oil eduction by pyrolysis of kerogens. The large volume of recycle gas is required in order that the necessary heat for eduction can be supplied without preheating the smaller volume of water gas from the gasification zone to such high temperatures as to bring about undue cracking of the educed oil. High-temperature gases are also undesirable because heat is unnecessarily wasted on the decomposition of mineral carbonates in the spent shale rock.
The combined eduction gases and product oil are then recovered from the eduction zone and separated as described more in detail hereinafter. When air is used as the oxidizing gas in the gasification zone, the retort off-gas will comprise a large proportion of nitrogen and will have a relatively low heating value. Where substantially pure oxygen is employed in the gasification zone, a high Btu retort off-gas is obtained which is much richer in hydrogen, and from which carbon dioxide can be separated to produce a 70-90 percent hydrogen gas stream suitable for use in a catalytic hydrofiner to upgrade the liquid shale oil produced.
A particularly desirable modification of my process involves the use of at least one retort operating with an oxygen gasification zone, in combination with at least one retort operating with an air gasification zone. The former retort or retorts produce hydrogen for hydrofining, while the latter produces low Btu heating gas to supply the thermal requirements of the combined retorting and gasification systems.
I am aware that combined shale retorting-gasification processes have previously been proposed, for example those described in U.S. Pat. Nos. 3,577,338 and 3,736,247. However, in these processes the raw shale is passed downwardly through the retorting zone and the gasification zone, while the necessary recycle gas for eduction (comprised of product gas from the retorting zone) is introduced at the bottom of the gasification zone and passed upwardly, first through the gasification zone and then through the eduction zone. Steam and oxygen are introduced at intermediate levels to effect gasification. I have found these procedures to be disadvantageous from several standpoints. Firstly, it is difficult to maintain a sharp separation between the eduction and gasification zones. Educed oil in the top of the retort tends to reflux downwardly into the hotter gasification zone where cracking may occur, and where there may be unconsumed oxygen. But even more importantly, I have found that it is disadvantageous to pass retort off-gas through the gasification zone, for two principal reasons:
Firstly, prior to reheating for recycle, this product gas has of necessity been cooled to condense out most of the vaporized retort hydrocarbons, and concomitantly water vapor is condensed, with the result that only a low equilibrium concentration thereof remains in the recycle gas. In the gasification zone it is desirable to maintain a high partial pressure of water vapor in order, inter alia, to shift the equilibrium of reaction (5) above as far as possible to the right. If sufficient steam is added to the recycle gas to achieve this objective in the above noted prior art processes, there would be excess gaseous heat carrying capacity, both in the gasification and retorting zones. Moreover, inordinate amounts of steam would need to be generated and condensed with each pass through the system, necessarily entailing large heat losses.
Secondly, the retort product gas contains about 2-10 volume percent of light hydrocarbons which are burned preferentially in the combustion-gasification zone. The only way, in this operation, that the coke on spent shale can be utilized is by reaction with steam and carbon dioxide. It has been found that the hydrocarbons in the recycle gas suppress the gasification reactions so that less than half of the coke on spent shale can be gasified in this manner. As a result, more than 10% of the recoverable energy in the raw shale is lost to the ash discard. These considerations appear not to have been properly evaluated in prior art gasification-retorting processes.
It is accordingly an important feature of my invention to introduce the eduction recycle gas into an essentially oxygen-free transition zone maintained between the gasification and eduction zones, and in which the spent shale has not yet been heated to above about 1100.degree. F. This permits control of the total volume of gas flowing into the pyrolysis zone so that the optimum gas to solids ratio can be obtained in this zone. The quantity of steam required in the gasification zone can be reduced and optimized since the steam requirement for gasification is less than the total volume of gas required for retorting. The temperature of the gas entering the pyrolysis zone also can be controlled and optimized. The temperature of the total gas (water gas plus recycle gas) entering the pyrolysis zone can be adjusted by controlling the temperature of the recycle gas leaving the recycle gas heater.
Another important consideration in shale retorting is the arsenic content of the produced oil. The presence of arsenic in the oil presents serious problems in the subsequent refining and use thereof. Most raw shales contain about 20-300 ppmw of As, and the educed oils may contain about 5-100 ppm thereof, depending not so much on the As content of the raw shale as upon retorting conditions.
The principal factors in retorting which appear to minimize uptake of arsenic by the oil are (1) providing a substantial water condensate phase in intimate contact with the condensed oil, and (2 ) minimizing the residence time of condensed oil in the retort at temperatures above the water condensation temperature, particularly in the range between about 200.degree. and 600.degree. F. My process provides an optimum combination of the foregoing conditions; the condensed steam from the gasification zone provides the desired water phase, and by virtue of solids-upflow, gas-downflow retorting, the retorted oil is rapidly swept downwardly, away from the hotter shale preheating and eduction zones. In solids-downflow, gas-upflow retorting however, the educed oil must be swept upwardly through the preheating zone, resulting in a refluxing condition and substantially increased residence time in the 200.degree.-600.degree. F zone of the retort. This condition, inherent in the above-noted prior art processes, provides increased opportunity for relatively water-soluble inorganic arsenic compounds formed during pyrolysis, e.g. As.sub.2 O.sub.3, to react with aromatic oil components, forming relatively oil-soluble, water-insoluble organic compounds, which tend to remain dissolved in the oil in preference to the water condensate phase.